The proportion of plastics in the automobile has risen greatly in recent years. Under the aspect of recycling plastics, however, endeavors are being made to limit the large number of plastics to indispensable few types. With this as background, it is understandable that polyolefins, particularly polypropylene, polypropylene copolymers and blends, with their broad spectrum of properties and good recycling capabilities, are preferably being used in the automobile sector.
Unfortunately, aside from their indisputably good properties, all polyolefin-based plastics, as described in the Encyclopedia of Polymer Science, second edition, 1985, volume 6, Ethylene Polymers (pages 383-522), Ethylene-Propylene Elastomers (pages 522-564) and volume 13, Propylene Polymers (pages 464-531), also have serious deficiencies, namely, their low free surface energy. The high hydrophobicity and crystallinity of polyolefins make the wetting and adhesion of less hydrophobic or hydrophilic materials difficult. This state of affairs become noticeable when the surface of these plastics is to be coated, printed, glued or foamed.
There has therefore been no lack of efforts to make the surface of polyolefin articles polar by special, subsequent treatments. The publication "Surface Treatments of Polyolefins" (Progress in Organic Coatings, 21, (1993) 269-284, J. M. Lane and D. J. Hourston) provides a survey of the most common methods of treatment for increasing the surface energy of polyolefins, namely
corona discharge; PA1 plasma treatment; PA1 flaming; PA1 chemical etching; PA1 fluorinating; and PA1 ozonization and photooxidative treatment. PA1 R.sup.2 groups are --(CH.sub.2).sub.p groups, in which p is a number from 2 to 6, PA1 R.sup.3 group is a known chain-length regulator, which is free of active hydrogen atoms, and PA1 n and m are the same or different and in each case have an average numerical value of not less than 3. PA1 R.sup.1 is an alkyl group, with the exception of the t-butyl group, with 1 to 8 carbon atoms, PA1 R.sup.2 is the --(CH.sub.2).sub.2 -- group, PA1 R.sup.3 is the C.sub.12 H.sub.25 S-- group, and PA1 Vapor jet conditions: PA1 pressure: 80 bar; PA1 temperature: 58.degree. C.; and PA1 distance from the test piece: 12 cm PA1 Position of the spraying jet: along a cutting line at an angle of 90.degree. to the sample surface; and PA1 duration of test: 20 seconds. PA1 rating: 0 (very good)-5 (deficient)
The most common pretreatment method is flaming. Monitoring this process creates problems, since basically it does not offer 100% reliability, as even small defects can cause severe adhesion disorders. Some risk of fire and the necessary clean room atmosphere signify additional expenses. The processing of milled, recycled material (rejected parts or scrap coated parts) is also a problem. Residual coating particles take up their position or are combusted during the flaming and cause distinct pinholes and craters to be formed, the avoidance of which requires additional technical effort.
The corona pretreatment is widespread particularly for the pretreatment of polyolefin films or sheets to bring about the adhesion to adhesives or printing inks. Since the adhesion effect apparently is based on unstable bonds and declines after a short time, it is necessary to coat soon after the corona process.
With the plasma pretreatment, local adhesion disorders of the coating can occur and are attributed to insufficient contact of the surfaces with plasma. This pretreatment has found only limited application because of the high cost of the method. Chemical etching, for example by the chromic acid dipping process, admittedly brings about a great improvement in adhesion. However, problems of treatment of contaminated water arise.
It is a common feature of all the methods that they must be carried out as an additional step in the production of the finished polyolefin article. Particularly in the automobile sector, elastomer-modified polypropylenes (TPO) have been used increasingly in recent years. They are on the market as reactor blends (see Kunststoffe, 82, (1992) 6, pages 499-501, Polypropylene Reactor Blends, H. Schwager) as well as extruder blends and are used as functional parts, such as bumpers, spoilers, sun visors, mirror housings, etc. With that, the desire for the problem-free coatability of such molded parts comes into prominence. A survey of the common methods employed at the present time for the pretreatment of polypropylene blends before they are coated is given in the article: "Problemlosungen beim Lackieren von Polypropylene-Blends" (Solutions for the Problems of Coating Polypropylene Blends), Ch. Gruner, B. Rapp, H. J. Zimmermann, Kunststoffe, 82, (1992), 9, pages 802-806. The solutions for the problem, described here, also always contain a physical-chemical treatment of the surface of the finished molded article and, with that, an additional step in the process before the actual coating.
It has now been found that the problems described are avoided, if special additives are incorporated during the processing of the polyolefins either during the compounding or directly during the injection molding over the melt before the molding.